Big Black Ghost Knife Fish hiding in aquarium plants surrounded by pebbles below

How Big Do Black Ghost Knife Fish Get? A Comprehensive Guide To Size And Growth

Exotic Aquarium Fish


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Wondering just how much room that mysterious Black Ghost Knife Fish will need as it grows? These striking creatures can reach an impressive 20 inches in length. Our article dives deep into understanding the full growth potential and care requirements for these unique freshwater fish.

How Big Do Black Ghost Knife Fish Get?

  • Black Ghost Knife Fish can grow up to an impressive 20 inches in length, making them a visually striking addition to any aquarium. This substantial size sets them apart as one of the larger freshwater fish species, captivating hobbyists with their unique appearance and behavior.
  • Providing these majestic fish with the right habitat is essential, as they require a tank of at least 100 gallons to thrive. The ample space not only accommodates their considerable length but also allows them to exhibit their natural swimming and gliding behaviors.
  • Proper care is essential for Black Ghost Knife Fish in a freshwater aquarium. These unique creatures have the potential to live a fulfilling life spanning 10 to 15 years when provided with the right conditions. This extended lifespan emphasizes the critical importance of maintaining an appropriate tank environment and ensuring a well-balanced diet to safeguard their health and overall well-being during their time in captivity.
  • To support their proper growth and overall health, a carnivorous diet is imperative for Black Ghost Knife Fish. These voracious eaters rely on live foods such as bloodworms and shrimp to meet their nutritional requirements, mirroring their dietary preferences in their natural habitat. Providing them with these protein-rich sources not only promotes growth but also helps maintain their vibrant appearance and vitality.

Keep reading to make your aquarium a perfect home for them!

Black Ghost Knife Fish Overview

A large Black Ghost Knife Fish swimming in a spacious freshwater aquarium.
Photo by Per Erik Sviland (cc).

The Black Ghost Knife Fish can grow up to 20 inches in length, making them a larger species of freshwater fish. They have a lifespan of 10-15 years and are carnivorous in nature, requiring a suitable diet for their species.

General species summary

Black ghost knife fish come from South America. They live in freshwater, not saltwater. They are special because they can make weak electric fields to find their way around and talk to each other.

These fish are not like others; they don’t have an anal fin or a caudal fin like most do.

These fish need a big home, with at least 100 gallons of water. This is because they grow very large, up to 20 inches long! People sometimes think these are small fish when they buy them.

But really, black ghost knife fish get really big and live for many years if cared for right. Their bodies are flat and long, which helps them slip through the water without being seen.

This shape also makes it easy for them to hide during the day since they like moving around more at night.

Appearance and size

The Black Ghost Knifefish is a standout in any fish tank. Its body is long and almost ribbon-like, with a sleek look that catches the eye. This electric fish moves by waving its thin fin on the bottom of its body, which makes it look like it’s gliding through water.

They are mainly black but can have white rings or spots.

As they grow, these fish can reach up to 18 inches from head to tail. Because of their big size, they need a lot of space to swim and be happy. A 100-gallon tank or larger is best for one Black Ghost Knifefish to live comfortably without feeling cramped or stressed.

Their size also means they’re more suited for advanced aquarists who can manage large aquarium setups.


Black ghost knife fish can live a long time, much longer than many other kinds of pet fish. With good care, they often reach 10 years old. Some even make it to 15 years if everything is just right for them.

Taking care of their water, giving them the right food, and making sure they have space to swim helps them thrive.

These fish need clean water and the right tank mates to stay healthy. It’s also important not to stress them out because that can make any fish sick. If you look after your black ghost knife fish well, it could be your underwater friend for many wonderful years.

Black Ghost Knife Fish Care and Habitat

A black ghost knife fish swimming in a well-oxygenated 75-gallon tank with suitable hiding places and sandy substrate.

Ensure a tank size of at least 75 gallons with clean and well-oxygenated water, suitable hiding places, and a soft sandy substrate for your Black Ghost Knife Fish.

Tank size and water parameters

Black ghost knife fish require a minimum tank size of 100 gallons, with a recommended size of 120-180 gallons to accommodate their large size and swimming habits.

Suitable tank setup

Black ghost knife fish need a spacious tank with plenty of hiding spots and open swimming areas. Here’s how to set up a suitable tank for them:

  1. Tank Size: Provide a minimum of 100 gallons for one black ghost knife fish.
  2. Substrate: Use smooth gravel or sand to prevent injuries to their delicate bodies.
  3. Decor: Include caves, driftwood, and plants to create hiding places and mimic their natural habitat.
  4. Filtration: Use a powerful filtration system to maintain water quality in the large tank.
  5. Lighting: Keep the tank dimly lit as these fish are nocturnal and prefer subdued lighting.
  6. Water Parameters: Maintain stable water conditions with a temperature between 75-82°F and a pH of 6.5-7.5.
  7. Tank Mates: Choose peaceful tank mates like discus that won’t compete for space or food.

Recommended food and diet

Black Ghost Knife Fish should be fed a primarily carnivorous diet in captivity to ensure their proper growth and health.

  1. Live foods such as bloodworms, shrimp, and feeder fish are highly recommended for their nourishment.
  2. Insect larvae and small crustaceans also make for suitable food options for these fish.
  3. Freeze – dried alternatives can be occasionally added to supplement their diet, ensuring they receive a balanced nutritional intake.
  4. Their diet in captivity should closely resemble what they eat in their natural habitat, including gastropods, insects, worms, and invertebrates.
  5. It is important to avoid feeding them plants or vegetable – based foods as they require a strictly carnivorous diet for optimal health and growth.

Behaviors and temperament

Black Ghost Knife Fish are territorial and may show aggression towards other fish, especially if they feel threatened or if their space is intruded upon. They prefer to be active at night and generally enjoy minding their own business.

While they can coexist with other non-electric fish in a community aquarium, it’s best to avoid keeping them with other electric fish due to their territorial behavior. Overall, they tend to be relatively peaceful and active when given ample space and suitable tank conditions.

Potential health concerns

The black ghost knife fish is prone to common freshwater diseases, with Ich being the most prevalent. Due to their lack of scales, they are particularly susceptible to this disease.

The aggressive nature of these fish doesn’t make them ideal for community tanks, as they may not get along well with other fish species. Keeping them in an appropriate tank size is crucial to prevent increased aggression and compromised health.

Breeding Black Ghost Knife Fish

The gender differences between male and female Black Ghost Knife Fish can be identified through their size, with males typically being larger than females. Breeding these fish can be challenging, as they require specific water conditions and habitat setup to stimulate breeding behaviors.

Gender differences

Female black ghost knifefish and male black ghost knifefish have different electrical frequencies, which can be used to tell them apart. Researchers found that females have a higher electrical frequency than males, making it possible to recognize their gender based on this difference.

Although these fish do not display strong physical differences between the sexes, their distinct electrical frequencies provide a reliable method for distinguishing male and female black ghost knifefish.

Breeding information

Breeding Information:

  1. Provide a suitable breeding tank with low lighting and fine substrate.
  2. Use caves or shelters for the female to lay eggs and for males to guard.
  3. Maintain water temperature around 79-82°F (26-28°C) and pH between 6.5-7.5.
  4. Introduce live or frozen foods to condition the pair for spawning.
  5. Watch for the male’s courtship behavior, including chasing and nudging the female.
  6. The female will lay eggs inside shelters, which the male then fertilizes.
  7. After spawning, monitor the eggs closely as parents may eat them; consider removing and raising them separately if needed.

Conclusion and Key Takeaways

In conclusion, Black Ghost Knifefish can grow up to 20 inches, making them one of the larger freshwater aquarium species. It’s important to provide them with a large tank, suitable hiding spots, and a carnivorous diet for their optimal growth and well-being.

Understanding their nocturnal nature and peaceful temperament is essential for creating a suitable habitat. Overall, these unique creatures can live up to 15 years when properly cared for in a spacious and enriched environment.


1. How big can a Black Ghost Knife Fish get?

A Black Ghost Knife Fish, known as Apteronotus albifrons, can grow up to 20 inches long in a big freshwater aquarium.

2. What helps Black Ghost Knife Fish grow well?

To help them grow, give them a variety of fish foods and make sure the water is clean by using things like a UV sterilizer.

3. Can I put other fish with my Black Ghost Knife Fish?

Yes! They can live peacefully with some other fishes like angel fish (Pterophyllum spp.), bettas, and neon tetras in your tank.

4. Do boy and girl Black Ghost Knife Fish look different?

Not really; they don’t show sexual dimorphism, which means you can’t tell males from females just by looking at their shape or size.

5. What should I feed my Black Ghost Knife Fish to help it grow?

Feed your knife fish flake food or small live food that is right for its nocturnal habits and growth rate – it loves eating when the lights are off!